Physics in the classroom
Lecture 7
Physics 304
Magnetism story
Once upon a time a knight in armor was riding through the
countryside, when he was suddenly snatched from his horse by
a strange force. The force held him pinned to a rock. He was
horrified to see the remains of another knight. He noticed that
the mysterious force held fast to the unfortunate knights armor,
but not his bones. So our brave knight managed to reach his
dagger and cut himself out of his armor, freeing himself.
Physics 304
Magnetism
We now know this strange force as magnetism, and the strange
rock as lodestone, a natural magnet.
Natural magnets were used for many years, particularly for
navigational compasses.
All magnets have north and south poles. When two magnets are
brought together, like poles repel and unlike poles attract.
Physics 304
Electricity + Magnetism = Electromagnetism
Electricity was also known for many years, but magnetism
and electricity were believed to be totally separate
phenomena.
Hans Christian Oersted discovered, while preparing a lecture in
fact, that there was a connection between electricity and
magnetism. This connection had a tremendous impact on things
ranging from electric motors and generators to a clearer
understanding of special relativity.
Physics 304
Changing magnets produce electricity
N
S
meter
Physics 304
Electricity produces
magnetism
All electrical currents produce magnetic fields. These fields are
usually very small. They can be magnified by wrapping
current-carrying wires into a coil. They can be further
intensified by inserting a piece of iron into them. This
arrangement is commonly called an electromagnet.
Physics 304
Electromagnet
Iron not magnet
battery
Physics 304
Electricity
Magnetism
So, moving magnets produce electricity and moving charges
(electricity) produce magnets.
Physics 304
Electric generators
Electric generators produce electricity by moving magnets relative
to coils of magnets, in much the same way we produced
electricity by moving a magnet in and out of a coil of wire.
Physics 304
Electric motors
In some sense electric motors are electric generators in reverse.
A typical electric motor operates by creating an electromagnet
with the incoming electric current. That electromagnet is
repelled by a permanent magnet inside the motor. This
repulsion produces the desired motion.
Physics 304
Transformer
No, not the toy that transforms from a robot into a flying
machine. A transformer converts electrical power at one voltage
and current to electrical power at another voltage and current.
Transformers are often found in residential neighborhoods.
These transformers convert the high voltage, low current
electricity in electrical power lines into low voltage, high
current electricity suitable for household use. Other examples
of transformers include the little square boxes that plug into the
wall and convert household electricity to electricity suitable for
small electrical appliances such as tape players or CD players.
Physics 304
More Transformers
Iron core
Primary coil
Notice the two coils of wire are
NOT connected. The power
goes from electrical power in
the primary to magnetic power
in the core and then into
electrical power in the
secondary.
Secondary coil
Physics 304
Power in = Power out
The idea that you can not get something for nothing seems to be a
recurring theme in physics.
In an ideal transformer, the electrical power that flows in through the
primary is the same as the electrical power that flows out through the
secondary.
This leads to a relationship between the number of coils in the
primary N1 , the voltage across the primary V1, the number of coils
in the secondary N2 and the voltage across the secondary V2.
V2 = N2
V1 N1
Physics 304
Example
A transformer with 100 coils on the primary and 10 coils on the
secondary is plugged into a wall outlet which has a voltage of
120 Volts. What is the voltage across the secondary?
N1 = 100
N2 = 10
V1 = 120V
V2 = ?
V2 = N2
V2 = N2 V1
rearranging
V1 N1
N1
V2 = 10 x 120V = 12V
100
Physics 304
Demonstrations
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Jumping ring
Magnet in a tube
Magnet, coil, galvanometer, bar
magnet.
Physics 304